My first Donzi and a few questions

[barstool]

I just bought a 98 21XLR bow rider. It's got a few performance issues with it.
!. It has a Mercruser 305 with a Alpha 1 out drive with 1.43 gears, a 17 pitch prop. This boat has been setting for several years, and had old gas in it.
2. This boat sat in the water for about 2 years and has a lot of dryed algie on the bottom of it. I will try to upload a pic of it.
3. With the above stated I tooker out on the water, just me and wife in it. On the gps would only run 36 mph at 4200 rpm tops, trimmed out. This boat only had 65 hrs on it when I got it, so made some runs to empty the tank, but first I put 2 quarts of Lucis in the tank, and topped it off with premium fuel. While running it it seamed to run smoother and started great. After 10 hrs I opened it up to see what top speed was and noticed that the tach was increasing rpm, I thought that the fuel conditioner was working so keep her wide open to see what it would do. The tach went to 5300 rpm, but no change of speed, or sound of engine. Then all at once the tach went back to 4200 rpm, and this never repleted the rest of the weekend.
I am going to take a hand tach to the lake with me this weekend to see if the factory tach is inaccurate. If it is I have a ss 19 pitch Apolo that I am going to put on it and see what see does.
If the tach is correct would the dryed algie on the hull slower down this much and take the rpms down with the added drag?
Short of putting in a new 502 what suggestions does anyone have, as I plan to bring her home this fall and polish the hull to get rid of the buildup it has.
What am I to expect top speed to be as I assume that the mid to hi 40's are achievable with this set up?
Thanks in advance for helping a newbie!

[VetteLT193]

the 19p prop will only reduce rpm's even more than they are now.

It may still be the gas... i would run the tank out and start fresh. The bottom isn't helping, but the pictures don't make it seem that bad.

The way the boat is configured, you are only looking at a top speed in the mid 40's. Your prop slip is around 20%, so if you can get the engine to spin up to 4800, you are looking at 42-43MPH

Have you checked compression of the cylinders?

[barstool]

the 19p prop will only reduce rpm's even more than they are now.
It may still be the gas... i would run the tank out and start fresh. The bottom isn't helping, but the pictures don't make it seem that bad.
The way the boat is configured, you are only looking at a top speed in the mid 40's. Your prop slip is around 20%, so if you can get the engine to spin up to 4800, you are looking at 42-43MPH
Have you checked compression of the cylinders?

Not yet.
I was going to run a couple of tanks of fuel thru her before I replaced the plugs. Just in case the old fuel might of coated and fowled the plugs. The rear bottom is much worse than the bow is, as I could get to that to clean it at the car wash.
The lady I bought this from stated that before her husband died that on several occasions that it went in the mid 40's, stated that she knows it went over 45.

[VetteLT193]

Well, there is a bunch of stuff to check. Fresh gas first, then I'd check the carb to make sure it's not gummed up (is it carb, or injected?), plugs, and compression check.

No matter how little hours are on it, it could still have low compression, or even be running a cylinder short if there is a bad plug or wire.

[zelatore]

CLEAN THE BOTTOM!!!

It doesn't look that bad in the pics, but if it sat for 2 years in the water, even if it was bottom painted the paint is shot by now and I can only imagine how bad the growth would be - even in fresh water!

A few years back I had a customer complaining that his 32' aft cabin wasn't running right - it wouldn't get on plane. One look, and we said 'call a diver and clean the bottom'. He insisted he'd had it cleaned 'recently' but it was obvious he hadn't done it in at least 6 months. Probably more. So we went on a test run with the owner - sure enough, it only hit about 15 knots at WOT. Took it back to the slip where we had a diver waiting. He cleaned it and we went back out. 24 knots. Thankyouverymuch, that will be $500.

It doesn't take much to cause a lot of drag. Cleaning is a PITA, but at least you can haul the thing out on a trailer. Pitty the poor soul who has to call a diver ever couple months :(

You can try some of the dedicated bottom cleaners if you're having a hard time getting it clean. On-&-Off is one that I've used before. They are acidic, so bring your rubber gloves!